An image formation method of utilizing the so-called scanner system as an exposure system for silver halide photographic materials is known. Therein, a photographic material undergoes the exposure based on image signals obtained by scanning an original, and therethrough forms therein a negative or positive image corresponding to the original image.
Various recording apparatuses in which the image formation methods of utilizing the scanner system are embodied are used. Therein, the so-called dot generator system prevails at present. Such a scanner-utilized recording apparatus uses as the light source for recording a glow lamp, a xenon lamp, a mercury lamp, a tungsten lamp, a light emitting diode and so on.
All the light sources cited above, however, are inferior in practicality because they have weak power and short life. In order to remove these drawbacks of the above-cited light sources, coherent laser beams, such as beams emitted from He-Ne, argon, and semiconductor laser devices, are used as the light source of a scanner system.
Various characteristics are required for photosensitive materials to which a scanner system using coherent laser beams is applicable. Since the exposure time of such a scanner system is particularly short, that is, in the range of 10.sup.-3 to 10.sup.-8 second, it becomes essential for the photosensitive materials to ensure high photographic speed and high contrast even under such a short exposure. In addition, highly sensitive photosensitive materials are of greater advantage when the output is reduced for lengthening the life of a laser tube. For obtaining satisfactory halftone dots, on the other hand, it becomes necessary to properly adjust the shape of a laser beam with a slit or the like. The shape adjustment causes a reduction of the laser output, and in order to cope therewith a photosensitive material having high sensitivity is required.
Moreover, recent graphic arts industry has been strongly desirous of an improvement in working efficiency, or speeding-up of working. Accordingly, the speeding-up of scanning and for reduction in processing time of a photosensitive material is comprehensively needed.
In order to meet the needs of graphic arts, it is desirable that the exposure device (scanner, plotter) be designed such that the scanning may be speeded up and high image quality may be achieved by increasing the number of scanning lines and by narrowing down the beam emitted therefrom. As for the silver halide photographic materials, on the other hand, it is desirable that they have high sensitivity and processing stability and can be developed rapidly.
Besides the photosensitive materials for scanner system, photosensitive materials for photograph-taking use are also required to form therein satisfactory halftone dot images and line original images. Therefore, it is desirable for these materials also to have high sensitivity, excellent processing stability and rapid development suitability.
For desirable reproduction of continuous tone images with halftone dots or that of line original images in the field of graphic arts, there is the need of an image-forming system which can provide photographic properties including ultra-high contrast (especially a gamma value (.gamma.) of at least 10).
As a method of achieving high contrast which is one of photographic characteristics, a lithographic development system making use of the so-called "infectious development effect" has been employed from old times. However, such a system has a defect that the developers usable therein are difficult to handle because of their instability. Therefore, development of another image formation system capable of providing the photographic characteristic of being ultra-high contrast through development with a sufficiently stable processing solution has been needed. Methods of embodying such a system are disclosed in, for example, U.S. Pat. Nos. 4,166,742, 4,168,977, 4,221,857, 4,224,401, 4,243,739, 4,269,922, 4,272,606, 4,311,781, 4,332,878, 4,618,574, 4,634,661, 4,681,836, 5,650,746. Therein, a silver halide photographic material of the kind which forms a latent images predominantly at the surface of the grains, and to which a hydrazine derivative is added, is developed with a stable MQ or PQ developer adjusted to pH 11.0-12.3, and thereby is produced a very high contrasty negative image having a gamma value greater than 10. According to those methods, the photographic characteristic of being ultra-high contrast and high sensitivity can be achieved, and the stability of the developer to aerial oxidation can be made higher by far than those of conventional lithographic developers since a sulfite can be added thereto in a high concentration.
However, conventional development systems use dihydroxybenzenes as developing agent, and therefore have some disadvantages from viewpoints of ecology and toxicology. For instance, hydroquinone is an undesirable agent because of its allergenic effect, and 1-phenyl-3-pyrazolidones are hard to undergo biological decomposition. In addition, sulfites added in a high concentration cause high COD (chemical oxygen demand).
On the other hand, enediols such as ascorbic acid are known to function as a developing agent, and deserve notice as the developing agent free from ecological and toxicological problems. For instance, U.S. Pat. Nos. 2,668,549 and 3,826,654 disclose that those compounds enable the image formation under a high alkaline condition such that the pH is higher than at least 12. In the image formation method of this type, however, the developability is rather low and the development proceeds slowly as compared with the method of using a dihydroxybenzene developing agent. That is, the image formation method of using enediols as developing agent has the problem of lack of rapid processing suitability. The term rapid processing as used herein refers to the processing in which it takes 15 to 60 seconds for the top end of a photosensitive material to travel from the insertion slit of an automatic developing machine to the exit of the drying part of the machine via the developing tank, the fixing tank, the washing tank and the drying part in succession.
Although raising the silver chloride contents in silver halide emulsions used in a photosensitive material is already known to be effective for speeding the progress of development, that is to say, for conferring rapid processing suitability on the photosensitive material, it causes the lowering of sensitivity and the aggravation of unevenness due to physical development. Further, it tends to cause a rise in fog density.